Recommended Siting of Green Infrastructure by Type in Allegheny County

About This Map:

The Pittsburgh region is served by a combined sewer, which collects, conveys, and partially treats both
stormwater and wastewater in the same infrastructure system. During periods of heavy rainfall, stormwater
can
exceed the capacity of combined sewers, which causes overflow into nearby rivers and streams. While these
combined sewer overflows (CSOs) mitigate upstream flooding, they release untreated wastewater into receiving
water bodies.

Improvements to “gray” infrastructure - pipes, pumps, storage, and treatment facilities - can increase the
capacity of the collection system to accommodate more severe wet weather events. Conversely, “green”
infrastructure includes features that reduce the stormwater entering the collection system by temporarily
retaining or diverting stormwater. Types of green infrastructure vary from completely natural systems, such
as
converting a parking lot to a park, to single purpose engineered systems, such as pervious paving.

A comprehensive green infrastructure strategy starts with identifying broad areas appropriate for green
infrastructure given variation in hydrologic (land use plus topography) and hydraulic (the capacity of
pipes,
pumps, and treatment) then applying a mix of additional criteria to target specific installations.

Technically feasible installations of green infrastructure include all possible structural installations at
their
maximum load ratio plus all possible non-structural installations. The technically feasible installations
are
identified through hydrologic and hydraulic models performed at the sewershed scale.

Cost optimal installations are those technically feasible projects that, taken together, minimize the net
cost of
our wet weather plans. These projects are identified by applying engineering economic analysis (e.g., cost
effectiveness analysis) to scale estimated project-level performance to sewershed or service area level. The
cost optimal set of installations include all possible cost effective installations when considering gray
plus
green strategies in balance.

Not all cost optimal installations are realistic. The realistic potential of green infrastructure considers
land
use in balance and recognizes any limitations to installations on private property, where property owners
significantly influence the feasibility of installation.